[jamesr]

The strong magnetic field effect you refer to has nothing to do with the spacecraft problem. This effect only becomes important in very strong fields, in many orders of magnitude stronger than you have in the solar wind out where we are, they are typically measured in nT = 10^-14Gauss, in FF we have fields of 1^9 Gauss, ie 23 orders of magnitude higher.

[jamesr]

mchargue wrote: This might be of interest. It talks about how even a small magnetic field can be used to repel the solar wind – comprising protons & electrons in a plasma. It notes a particular effect of the plasma interacting with the small magnetic field that causes a repulsion effect much larger than what was originally expected.

It may be of interest & utility as so much of the FF is based on the interaction of plasmas with magnetic fields.

Pat

Magnetic sheild article

http://www.newscientist.com/article/mg20727701.300-shields-up-force-fields-could-protect-mars-missions.html?DCMP=NLC-nletter&nsref=mg20727701.300

I find it quite strange no-one had considered this earlier. However the wording of the article seems a little strange

The Larmor radius of the particles in that weedy magnetic field is about 120 millimetres: they should have smashed right into the magnet. But they didn’t

This seems to suggest a weak field results in a small larmor radius. It is the other way round: r=mv/qB, where v is velocity perpendicular to the field.

for protons; taking the component of the velocity as 100km/s (typical solar wind) and r=120mm gives the B field they used as ~ 100Gauss, which is orders of magnitude stronger than the Earth’s.

[jamesr]

Brian H wrote:
So the temp ref in the article is wrong? Interesting.

In the core of a star there are so many photons that cause the radiation pressure that normally balances the gravitational compression. It would only take a slight imbalance of a few of the highest energy photons being used up to form electron/positron pairs to start a cascade. Once the gravitational collapse starts, enormous amounts of potential energy are converted to kinetic. Heating the layer of the star in the process, and so implosion turns to explosion. Depending on the size of the star this could just eject the outer layers or if the collapse proceeds further triggering fusion of heavier elements and you have a huge supernovae.

See: wikipedia – Pair instability supernova

[jamesr]

Brian H wrote: Here’s an article that has set me wondering: http://www.sciencenews.org/view/generic/id/50258/title/Star_outweighed_any_known_in_Milky_Way

The relevant quote:

Theory predicts that any star heavier than the equivalent of 140 suns blows up in a very special way. Photons produced at the core of such a star provide an outward pressure that resists gravity’s inward pull. But when the core temperature exceeds about a billion kelvins, the photons suddenly become energetic enough to annihilate each other and produce pairs of electrons and positrons.

Is such pair instability a possibility in the FF plasmoid, since temps would be in that range?

Normally for electron-positron pair production, in Compton scattering, you need a photon with at least 1.02MeV (ie twice their rest mass energy of 0.511MeV). I believe, for two photons to annihilate you need to take into account higher order effects of QED, as normally photons do not interact directly with each other, only with charged particles. But the end result is the same, you need a total of 1.02MeV available to create the pair.

Since we are taking about temperatures of 50-100keV you will not really have any x-ray photons with that kind of energy. There may be the odd one created from the energetic fusion products, but nothing significant I would expect.

[jamesr]

Aeronaut wrote: Most of that went over my head, James. Am I correct in understanding that the ion heating effect you described could possibly apply to the describing and predicting the plasma temperature/ energy level at any point during the axial phase?

most of the heating in the axial phase will still, I think be due to the compression (roughly adiabatic) and and resistive heating. but when you get to the point where the field get so twisted, you can get a release of the energy that is stored in those twists by the field reconfiguring (snapping & reconnecting) with another part of the field to effectively undo the twists.

This heating mechanism though does not accelerate the particles adiabatically, instead you get some ions/electrons with much higher energies.

[jamesr]

Brian H wrote: Second, temperature is measured in electron volts, and tiny-ness is FF’s friend

I’m not quite sure how you think the units you measure something in suddenly changes the physics of what going on, but I get your general point.

On a separate topic, one thing I came across today in a talk about astrophysical plasmas, was that in magnetic reconnection (ie where a a twisted or opposing field changes topology), the release of magnetic energy to the particles causes them to be accelerated to supra-thermal speeds. This mechanism of heating ions & electrons as the plasmoid forms could play a crucial role in the dynamics. In that you cannot consider the plasma in the plasmoid as it forms to have a specific temperature, as such, because the dynamic have forced it well out of a Maxwellian thermal distribution. The density and collisionality of the plasma probably means it will thermalise again within a very short time, so it may be insignificant, but then again this seed population of fast ions could propote more fusion, or on the other hand a population of fast electrons created by this process could promote extra bremsstrahlung & have a negative effect.

It seems the more I learn about plasma physics the less you can predict by simple analysis. The only way to really tell is to do a non-linear simulation to test the hypothesis, and even then it only tells you about the particular instabilities/modes that can be accounted for in whatever simplified model you are using.

[jamesr]

Brian H wrote: but the toroid and filaments must be re-directing the current away from the anode once they form.

Indeed… the pinch is characterised by a sharp drop in current to around half the peak value over a few 10s of ns. This is assumed, I think, to be when the magnetic field changes topology and the separate plasmoid is formed at the heart of the focus. Although the exact mechanism for this is little understood.

[jamesr]

Brian H wrote:
Current causes heat; the only link in a circuit between the cathodes and anode is the plasma. It is still not clear to me that that circuit “closes”, resulting in current through the anode.

That’s how a DPF works! The current starts to flow when the plasma arcs across the base between the cathodes & anode. This radially inward current sheath of plasma creates a magnetic field around the cylindrical axis. The cross product of the current and field causes a force along the axis, sweeping the sheath of plasma down to the end where it filaments & and goes kink-unstable forming the focus.

Its all about the current and how we get that up to the 3MA or so needed to put enough energy into the magnetic field at the focus.

[jamesr]

Brian H wrote: Color me confused, but I thought the primary heat source/problem to be addressed was the plasmoid’s output, not the application of charge to the plasma through the birdcage.

Most of the energy from the plasmoid will be in the form of X-rays or the electron & ion beams – and hopefully more than was used to make it due to the excess of some fusion 😉 But most of the direct heat, will be from the resistive joule heating due to the current flow in the anode & cathodes. The total surface area of the ~12 cathodes is much larger than the anode so their proportion of the heating problem will be less – hence why we are concentrating on the anode.

A proportion of the X-ray energy will inevitably end up as heat, but that will be distributed throughout the volume of the anode and the whole spherical shell of components in the firing line.

[jamesr]

Given the pulsed nature of the device, I’m thinking any stratified surface layers would be problematic unless they had very closely matched thermal expansion coefficients.

In the central well of the anode where the electron beam hits you could consider a special coating in this area (such as 20um of tungsten), but for the main shaft of the anode, and cathodes where the current is flowing then you want as low a stress due to thermal & pinch forces as possible. The overall cooling aspect I think is manageable if you pump enough helium through. It is the fatigue and cracking due to many cycles that I’m beginning to get more concerned about.

If you carefully graduated the electrical conductivity near the surface, in the opposite sense to the skin current’s normal preference so you got a more even current distribution deeper through the top 1mm of material it may help.

[jamesr]

I have been thinking any surface layer will need to be much thicker & more robust than a few nm skin.

If you compare with the materials & heat fluxes for tokamak divertors (the area at the bottom of the tokamak where the magnetic field sweeps any plasma that escapes the confinement down to collide with & dissapate its energy). They are either carbon fibre composite (CFC) or tungsten. We would not really want tungsten as it would absorb too much of the X-Rays, but the CFC materials they use could be a possibility, and can cope with ~20MW per m^2, and transient peaks of 500kJ/m^2 in 0.1ms or 5GW/m^2.

Even if we had a 0.1mm surface conductive/protective layer the differential thermal & pinch force stresses between it and the beryllium bulk would probably just make it all crack & fall off.

Another thought is to dope the beryllium to form an alloy with better overall electrical conductivity. After all Beryllium is pretty good anyway from most other aspects so just tweaking the properties slightly to optimize the thermal and electrical properties is probably the easiest option (although still a few years of research).

[jamesr]

Henning wrote: Found that article on “Next Big Future”: Rolled Up Capacitors Have Double the Energy Density of Flat Capacitors

Maybe we should also think about how to improve capacitors. The size of capacitors doesn’t matter for focus fusion, but this might improve inductance and resistance, which does affect current rising time.

I thought most capacitors were rolled up anyway??? certainly if you look at any circuit board or power supply with cylindrical caps standing up on it they will be rolled up.

For better capacitor energy storage capacity you want to use something like Electric double-layer capacitors, which have a massively increased surface area.

But for our application they can’t handle the high voltages. To have a capacitor able to charge to 45kV without the dielectric material breaking down restricts your choice somewhat.

[jamesr]

vansig wrote:

The trick will be arranging that power circuitry to deliver the constant 50/60Hz 3phase output, as the pulse rate goes up and down to match power demand from the local/national grid.

that’s just about like keeping a flywheel at constant speed

The simple solutions are the best….

I’m at Culham (the site of the JET & MAST tokamaks) at the moment – they have a couple of huge flywheels. Although theirs are used to give a pulse of output rather than constant supply.

[jamesr]

I’m glad it will be able to proceed without any more hold-ups. However the reference saying some of the money will be coming out of the current (FP7) EU research budget will inevitably mean less available for other research.

In the grand scheme of things though the 10billion or so going to ITER over many years is a drop in the ocean compared to the size of the commercial energy market. Its small even in terms of public finances considering the cost is spread amongst lots of countries (creating jobs & tax revenue back in the process). If industry put even a fraction of a percent into research they could build 10 ITER size projects without even noticing.

People (ie politicians, campaign groups etc) keep on going on about how important energy is, but nobody is putting money where there mouth is. We really need to get to the point where the energy industry invests even 2% of its turnover in R&D rather than the tiny fraction it does now.

[jamesr]

For the plasmoid at high temperature the dominant radiative cooling mechanism is via bremsstrahlung. The formula for which is approximated as:

P(br) = 1.7E-38*Z^2 *n_e * n_i *sqrt(Te) in W/m^3

where n_i, n_e are the ion & electron number densities in per m^3, and Te is the electron temperature in eV.

Taking the effective Z as 2 (combination of hydrogen & boron but with higher concentration of hydrogen) and ne =Z*n_i = 1e26/m^3 and Te= 1e9K = 80keV

Then I make P(br) ~ 4e17 W/m^3 for the plasmoid.

Multiplying this by the volume of the plasmoid (~10um diameter) an saying it lasts for 100ns gives only 4e-5J which seems way too small???

For the bulk plasma at a few thousand degrees the radiation comes from recombination and line radiation which is much harder to work out… I’ll try and find an estimate.

You also obviously have this hot, few thousand degree plasma in direct contact with the electrode surfaces – this I would expect to dominate everything.

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